Full-frame or frame transfer CCD image sensors can contain electrical and optical asymmetries in the pixel depending on the wavelength and incident angle of the light. When a color version of the device is coupled to a camera lens, the asymmetries cause mismatches in the amount of cross talk between adjacent pixels, which are of a different color, and varies across the array as the camera lens will introduce a range of light ray angles depending on the pixel position in the sensor. This is referred to as a positional hue shift or ‘color cast’.
Electrical asymmetries occur as a result of the electrostatic structure in the silicon that defines a pixel. Depending on the incident light angle, the position of the light shield aperture, and where signal electrons are created in the silicon, crosstalk from one pixel to another and may more easily occur in one incident light angle direction than another, creating the hue shift.
Optical asymmetries result due to a mismatch in the transmission properties of a multi-gate architecture. In particular, sensors using a combination of gate materials, such as polysilicon and indium-tin-oxide (ITO), will be more sensitive to optical asymmetries as the transmission properties of the gates become more disparate. As the incident light angles increase, the overlying light shield or other structures introduce a shadowing effect of the underlying gate materials causing preferential transmission through one type of gate material than another, creating the positional color hue shift effect. Any amount of optical asymmetry exacerbates the electrical asymmetry.
Consequently, a need exists for improving the optical symmetry of the pixel and, in particular, to improve the optical symmetry when using a combination of different gate materials. The present invention addresses this need.